In the staunchly conservative districts that most House Republicans inhabit, playing ball with President Barack Obama on taxes and the debt means tempting a primary opponent in the next election. The threat of a challenge from the left that might come from digging in, on the other hand, is almost nonexistent for most members.

As a matter of pure political self-interest, the post-election debate within the GOP about how to broaden the party’s appeal and avoid another Romney-esque debacle in four years is irrelevant in this quarter of GOP politics. For the overwhelming majority of House Republicans, the largely white, resolutely conservative electorate that Mitt Romney relied on — excessively, as it turned out — is all they need to ensure reelection.

That inescapable fact could be the single biggest obstacle to Obama’s second-term agenda. The same dynamic will undoubtedly apply when it comes to immigration reform, guns or energy. Forget breaking filibusters in the Senate; the House Republican Conference is what will keep Obama awake at night at least for the next two years.

The numbers tell the story. According to the Cook Political Report, the widely respected political handicapper, just six Republicans – around 3 percent of the House GOP Conference — will occupy districts whose overall voter makeup favors Democrats. That figure is down from 22 Republicans that resided in such Democrat-friendly districts in 2012.

That unusually high number of House Republicans occupying deeply red districts has intensified the fear of a primary — not general — election threat. And that means no deals with a president, who in most cases, lost those members’ districts resoundingly.

The phenomenon isn’t new. For decades, many Republicans — and for that matter Democrats — have found themselves locked into districts where they’re beholden to their party’s electorate. But it will be heightened in 2014.

One of the Republicans in a deeply conservative district, South Carolina Rep. Jeff Duncan, said he has no doubt the voters he represents want him to oppose higher taxes as part of any fiscal cliff deal.

“I believe if you do what you said you were going to do, if you vote with integrity, if you do what you told voters you were going to do, reelection will take care of itself,” he said. “People who have contacted me are asking me to stand firm on what really matters, which is the spending side of the equation.”

The polarization was exacerbated by the just completed, once-a-decade redistricting process. Both parties — but particularly Republicans, who swept control of statehouses across the country in the 2010 conservative wave — redrew district lines to shore up House members politically.